Our ability to hear well is key to our communication with the world around us. As in most mammals, loss of hearing occurs with age in humans with debilitating results. However, hearing deficits are becoming increasingly more common in younger people due to voluntary or involuntary exposure to noisy environments. Loss of inner hair cells (IHCs) and spiral ganglion neurons (SGNs) are the hallmarks of age-related and noise- induced hearing disorders. Appropriately, efforts are underway to restore hearing by replacing both of these cell types using stem-cell based approaches. Several decades of electrophysiological studies have shown that SGNs are functionally heterogeneous, so restoring the full range of normal hearing by replacing lost or damaged neurons will require careful consideration of different SGN subtypes. This is especially important given that a subset of SGNs with low spontaneous firing rates has been found to be particularly vulnerable to noise-induced damage. Unfortunately, little is known beyond the spontaneous firing rate-based classification regarding heterogeneity of Type I SGNs, which are the only ones that innervate IHCs. Notably, no molecular markers are known for any biologically relevant bases of Type I SGN classification. This has been a major roadblock, not only in investigating the significance of SGN heterogeneity for auditory coding, but also in generating the precise subpopulations lost after noise trauma. Here I propose to define the molecular and anatomical correlates of SGN heterogeneity and to examine the functional significance of different SGN subtypes. Specific aim 1 involves use of next generation sequencing and single-cell transcriptional profiling tools to generate transcriptomes of individual SGNs of postnatal day 20 mice, which will be used to identify transcriptional clusters that correspond to biologically relevant SGN subtypes. This aim also seeks to understand the significance of the identified SGN subtypes for hearing in mice by acutely and selectively silencing them. Specific aim 2 focuses on determining the anatomical correlates of SGN heterogeneity and seeks to answer a longstanding question in the field- are the cohort of SGNs innervating a single IHC diverse in their positioning within the ganglion and molecular profiles? To accomplish this, a transsynaptic viral tracing approach will be used to sparsely label individual IHCs and their innervating SGNs with GFP in mice. Results of these experiments should vastly improve our understanding of SGN heterogeneity independent of their apico-basal position, and promise to open new doors to investigation of auditory coding at the level of SGNs. Furthermore, knowledge of molecular fingerprints of SGN heterogeneity should facilitate further study of development of different SGN subtypes, which will be of benefit to ongoing efforts in the field to restore hearing by replacing lost SGNs.